1. Field of the Invention
The present invention relates to an antenna matching circuit, an antenna matching module, an antenna device and a wireless communication device that provide UHF-band communication, for example.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2011-55258 discloses a multiband antenna device that is used in a cellular phone terminal, for example. FIG. 21 shows the antenna device of Patent Japanese Unexamined Patent Application Publication No. 2011-55258.
FIG. 21 shows an example of a low-band-side or high-band-side variable matching circuit. This variable matching circuit includes a variable reactance unit RC and a matching unit M. The variable reactance unit RC includes a parallel circuit consisting of an inductor La and a capacitor Ca and this parallel circuit is connected in series with a base part of an antenna element 20. The matching unit M includes a parallel circuit consisting of an inductor Lb and a capacitor Cb and this parallel circuit is connected in parallel between a feeder circuit 10 and the variable reactance unit RC.
As shown in FIG. 21, the bandwidth of the antenna can be increased by actively switching the reactance generated by the variable reactance unit.
However, due to the demand for size reduction of wireless communication devices in recent years, the space for the antenna in antenna devices is becoming ever more restricted. Together with this, there is a tendency for the input impedance of antennas to become increasingly lower. When the variable reactance unit RC and the matching unit M include reactance elements and a switch, power loss due to the switch relatively increases as the impedance of the antenna element 20 becomes lower.
Even if the reactance is switched using a switch after matching the impedance of the antenna element using a reactance element (after increasing the impedance of the antenna element) in order to avoid this increase in loss, it is not possible to change the resonant frequency of the antenna over a large range in such a case. Furthermore, since the impedance cannot be (simultaneously) converted across a wide bandwidth when the impedance is changed using an inductor, a capacitor or the like, the loss due to a switch increases at the ends of the frequency band in which matching is being attempted.